The present invention relates to a hot melt moisture cure adhesive composition well suited for bonding porous substrates such as fabric, methods of adhesively bonding substrates and corresponding articles. The adhesive composition exhibits a combination of flexibility, high green strength, low molten viscosity and high heat resistance. In view of these properties, the adhesive is particularly amendable to hot melt adhesive application techniques wherein small individual discrete deposits of adhesive are applied to a substrate. The adhesive composition prepolymer is preferably the reaction product of at least one polyol, at least one polyisocyanate and at least one high molecular weight thermoplastic polyurethane lacking hard segments.
Reischle et al., U.S. Pat. No. 4,585,819 issued Apr. 29, 1986 and DE-A-32 36 313 relate to an adhesive comprising an isocyanate prepolymer, a low molecular weight ketone resins, and optionally a thermoplastic polyurethane or polyester. Exemplified is a thermoplastic polyurethane formed from 404 grams of polyester diol based on adipic acid 1,4 butane diol and ethylene glycol with an OH-number of about 55 and a molecular weight of about 2,000, 20 grams of 1,4-butane diol and 109.5 g of 4,4xe2x80x2-diisocyanatodiphenyl methane. The inclusion of 1,4-butane diol contributes hard segments in the thermoplastic urethane component.
Brauer et al., U.S. Pat. No. 5,036,143 issued Jul. 30, 1991 teaches a high green strength curable urethane adhesive composition of a polyol, an organic isocyanate compound, a thermoplastic polyurethane which is soluble in methyl ethyl ketone, and optionally a solvent for controlling the percent solids and viscosity of the composition. The most preferred TPU is Millathane E-34. Other suitable TPU""s are described in U.S. Pat. No. 3,043,807 issued Jul. 10, 1962, incorporated therein by reference. The 100% solids urethane adhesives are prepared from liquid reactants. The compositions are 100% solids in that the compositions do not contain solvents. However, due to the low melting point of the reactants, such compositions are flowable at room temperature. The adhesive compositions have a relatively high green strength for a liquid, per se. However, the compositions have relatively low green strength in comparison to thermoplastic solids wherein the initial green strength is obtained by the increase in modulus obtained by cooling the composition from a liquid state to a solid state. Further, in the case of bonding fabrics, liquid adhesive composition have a tendency to penetrate too far into the fabric resulting in poor bond strengths, undesirable stiffening of the fabric upon subsequent cure of the adhesive, or penetration through the fabric resulting in adhesive build-up on the equipment.
Takada et al., U.S. Pat. No. 5,115,180 issued Oct. 13, 1992 teaches a moisture curing hot melt adhesive comprising a blend of urethane prepolymers. The first urethane prepolymer comprising hard and soft segments as well as other specific properties, whereas the second urethane prepolymer comprises a soft segment moiety and terminal isocyanate group which is prepared in a specific manner. The exemplified adhesive compositions have a relatively high viscosity, greater than 1xc3x97106 cps at 120xc2x0 C.
More recently, Krebs, U.S. Pat. No. 5,994,493 issued Nov. 30, 1999 stated that, xe2x80x9cFor formulations which are intended to have high initial strengths and to be applied to porous materials, 5 to 40% by weight of the thermoplastic polyurethane has to be added. Unfortunately, this increases the melt viscosity with the result that these adhesive can only be applied by spraying at very high temperatures.xe2x80x9d Krebs describes reactive hot melt adhesives having a viscosity of less than about 8000 mPas at 150xc2x0 C. consisting of a polyurethane prepolymer terminated by reactive isocyante groups which is the reactive product of based on liquid polyether or polyester polyol having certain properties, certain polyol alkoxylation products having a molecular weight below 1000 and at least one diisocyanate.
Although low in viscosity, the compositions of Krebs et al. tend to be stiff and exhibit short open times. Accordingly, industry would find advantage in hot melt moisture curing adhesive compositions having an improved balance of properties, particularly for fabric bonding.
The applicants have discovered a hot melt moisture cure adhesive composition exhibiting a synergistic combination of low viscosity and high green strength that is particularly useful for fabric bonding. The present invention relates to an article comprising a permeable or porous substrate bonded to at least one substrate with a hot melt moisture cure adhesive composition having a viscosity of less than about 60,000 cps and preferably less than about 30,000 cps at 250xc2x0 F. The low viscosity property of the adhesive composition is advantageously combined with high green strength, as reflected by the peel resistance and/or storage modulus, Gxe2x80x2. The storage modulus (Gxe2x80x2) of the adhesive composition after about 100 seconds of cooling ranges from about 1xc3x97104 dynes/cm2 to about 1xc3x97106 dynes/cm2. Further, the adhesive composition typically resists a bond displacement of about 20 mm for at least about 1 minute, and preferably for 10 minutes or longer. Advantageously, the composition of the present invention exhibits a higher storage modulus, (Gxe2x80x2) in comparison to other hot melt moisture cure adhesive composition having about the same complex modulus or stiffness. The composition is flexible, having a complex modulus (G*) ranging from about 100 seconds of cooling ranges from about 1xc3x97104 dynes/cm2 to about 1xc3x97107 dynes/cm Additionally, the composition preferably exhibits high heat resistance, having a peel adhesion failure temperature greater than 250xc2x0 F.
The hot melt moisture cure polyurethane composition is prepared from about 40 wt-% to about 80 wt-% of at least one polyol, from about 5 wt-% to about 50 wt-% of at least one thermoplastic polyurethane that is substantially free of hard segments and having a molecular weight (Mn) of greater than about 10,000 g/mole, and at least one polyisocyanate present at an equivalent ratio of isocyanate to hydroxyl ranging from about 1.2 to about 10. The polyol component is relatively low in molecular weight and in the case of fabric bonding, preferably amorphous.
In another embodiment, the present invention relates to a method of bonding a permeable or porous substrate, such as fabric, comprising coating the low viscosity, high green strength hot melt moisture curing adhesive composition. The adhesive composition can advantageously be applied at low application temperatures ranging from about 200xc2x0 F. to about 300xc2x0 F. The adhesive composition is well suited for gravure roll coating, pattern coating, and particularly porous coating hot melt adhesive application techniques, wherein small individual discrete deposits of the adhesive are applied to a substrate. Particularly in the case of porous coating, adhesive build-up on the die, known as xe2x80x9croll upxe2x80x9d does not occur.
Thermoplastic polyurethane is preferably prepared in situ during preparation of the hot melt moisture cure adhesive composition. At least one polyol is polymerized with at least one polyisocyanate such that a thermoplastic polyurethane having terminal OH groups and a molecular weight greater than 10,000 g/mole is formed. A second polyol having a molecular weight of less than about 5000 g/mole is added followed by a second addition of polyisocyanate. In preferred embodiments, the second polyol is the same polyol as employed in the preparation of the high molecular weight thermoplastic polyurethane.
The adhesive composition is particularly useful for hot melt application techniques that apply discrete individual deposits of adhesive to a substrate such as engraved roll coating, pattern coating and in particular the Nordson Porous Coat(trademark) System, as described in U.S. Pat. No. 5,750,444; incorporated herein by reference. In comparison to conventional slot-coating and spray techniques for applying hot melt adhesives, methods that apply discrete individual deposits of adhesive offer some distinct advantageous. In one aspect, such methods typically employ lower amounts of adhesive per area of substrate being coated. This not only reduces the total adhesive consumption and thus, the total adhesive cost, but more importantly in the case of fabric bonding, reduces undesirable properties contributed by the adhesive such as reduced breathability and increased stiffness. As used herein, xe2x80x9cfabricxe2x80x9d refers to a woven or nonwoven textile material formed from fibers of such materials as cotton, wool, polyester, polyamide, polyurethane, as well as various polyolefins. The majority of thermoplastic adhesive compositions do not exhibit good moisture vapor transmission properties. Accordingly, if a breathable fabric is coated with such adhesive compositions, the breathability is greatly diminished by the hot melt adhesive coating.
The Nordson Porous Coat(trademark) System employs a slot die to dispense hot melt adhesive in a fibrous or porous adhesive matrix by means of metering pumps feeding independently and separately controlled slot dies. The adhesive pattern or matrix is typically contact coated directly onto a substrate. The adhesive can be applied at very low coating weights ranging from about 0.075 to about 30 grams per square meter. For breathable composites, the majority of the surface area of the substrate is not coated with adhesive. Hence, the coating weight typically ranges from about 1.5 to 12 grams per square meter.
The adhesive composition of the present invention has been found to be particularly suitable for porous coating in view of the fact that the adhesive is substantially free of a process phenomena known as xe2x80x9croll upxe2x80x9d. xe2x80x9cRoll upxe2x80x9d occurs when the adhesive mitigates up the lip of the die and collects on the shelf. The adhesive build-up subsequently releases onto the substrate yielding an undesirable coating. Roll up is surmised to be caused by the adhesive exhibiting a higher cohesive strength than adhesive bond strength to the substrate being coated.
Pattern coating involves extruding a hot melt adhesive through a rotating screen that in turn deposits the adhesive directly onto the substrate to be coated. Alternatively, the screen may deposit the adhesive first onto release paper and then transfer coat the adhesive onto the intended substrate. The screens typically used in this process range from about 15 mesh to about 195 mesh, preferably from about 30 to 60 mesh, and more preferably from about 30 to 40 mesh. The 30 to 60 mesh screen results in individual hot melt adhesive deposits ranging in mass from as little as about 5xc3x9710xe2x88x925 grams per adhesive deposit to about 0.05 grams per adhesive deposit.
In the case of engraved roller applications, the adhesive is provided in a molten form in a reservoir. A roller that has been engraved with depressions is positioned such that as the roller revolves, a portion of the roller contacts the molten adhesive. Excess molten adhesive that is not present within the depressions is typically scraped off the roller. The roller is then subsequently contacted to a substrate, depositing the adhesive within the engraved depressions onto the substrate. The temperature of the roller may be controlled by a heating and/or cooling means. In any event, the roller typically reaches the temperature of the molten adhesive ranging from about 90xc2x0 C. to about 180xc2x0 C. For heat sensitive substrates such as low gauge polyolefin films, preferably the temperature of the roller does not exceed about 160xc2x0 C. and more preferably the roller is maintained at a temperature of about 140xc2x0 C. or less and more preferably at a temperature ranging from about 90xc2x0 C. to about 125xc2x0 C.
After coating by any of these methods, the number of individual hot melt adhesive deposits per area of substrate ranges from as little as 1 per square inch to as many as about 600 per square inch. The total mass of adhesive per area ranges from as little as about 5 g/m2 to about 50 g/m2. However, higher amounts may be useful for some applications requiring very aggressive adhesion and wherein the breathability is not of concern.
Preferably, the adhesive is coated directly onto the substrate to insure that a mechanical rather than merely a surface bond is formed with the fibers of the fabric. Transfer coating is generally less desirable, particularly since most of the heat has dissipated by the time the adhesive is transferred onto the substrate. However, transfer coating may also be suitable, provided the open time of the adhesive is amenable to forming bonds of sufficient strength.
Although, the adhesive composition is particularly amenable to gravure roll coating, pattern coating, and porous coating, the composition may also be applied with other application techniques such as spray, slot coating, extrusion coating, roll coating and melt blown hot melt application techniques.
The hot melt moisture cure adhesive composition is substantially solid, having a high viscosity at room temperature. The viscosity is typically at least about 1xc3x97106 cps at 25xc2x0 C. Further, the glass transition temperature (Tg) ranges from about xe2x88x9240xc2x0 C. to about 10xc2x0 C. and preferably from about xe2x88x9220xc2x0 C. to about 0xc2x0 C. The open time of the adhesive composition ranges from 5-10 seconds to about 10 minutes or longer, depending on various application variables such as coat weight, application temperature, method of application, etc. When porous coated, the open time is surmised to be about 5 seconds.
The hot melt moisture cure adhesive composition may be applied at temperatures ranging from about 200xc2x0 F. to about 400xc2x0 F. However, in order to be suitable for direct coating onto heat sensitive substrates, the adhesive composition must have a low viscosity at a temperature less than about 300xc2x0 F., more preferably less than about 275xc2x0 F., and most preferably ranging from about 200xc2x0 F. to about 250xc2x0 F. Low application temperatures are also preferred in order""to minimize thermal degradation of the adhesive and for worker safety considerations.
The viscosity of the adhesive composition of the invention at application temperature is less than 500,000 cps and typically less than 100,000 cps. For porous, pattern and engraved roll coating the viscosity is preferably less than about 60,000 cps, and more preferably less than about 30,000 cps, and in some embodiments less than 15,000 cps. It is desired that the molten viscosity be as low as possible at as low of a temperature as possible without reducing the storage modulus and complex modulus after about 100 seconds of cooling below the targeted range. At too high of a viscosity, higher application temperatures are needed that may cause heat deformation of the substrates being bonded. At too low of a viscosity, however, the adhesive composition seeps into the fabric causing undesirable stiffness and blocking as a result of the adhesive striking through the substrate.
The moisture in the substrates and atmospheric moisture are normally sufficient for setting, although the reaction may be accelerated by additional spraying with water and/or media containing polyols or catalysts.
The adhesive composition of the present invention obtains high green strength without becoming too viscous for the application or without becoming too stiff for good wet out and good adhesion. In the present invention, the complex modulus, (G*), represents the relative stiffness of the composition. The complex modulus is the combination of the viscous modulus and elastic modulus component as determined by Dynamic Mechanical Analysis (DMA). The higher the G*, the stiffer the material. Further, the lower the tan xcex4, the more elastic and creep resistant the material.
The adhesive composition of the present invention exhibits a complex modulus of at least about 1xc3x97104 dynes/cm2 after about 100 seconds of cooling. Preferably, the complex modulus is greater than about 5xc3x97104 dynes/cm2, more preferably greater than about 1xc3x97105 dynes/cm2 and most preferably greater than about 2xc3x97105 dynes/cm2 after about 100 seconds of cooling. Particularly in the case of fabric bonding, the adhesive composition remains flexible after curing and thus, exhibits a complex modulus of less than 1xc3x97107 dynes/cm2, preferably less than about 5xc3x97106 dynes/cm2, and most preferably less than about 2xc3x97106 dynes/cm2.
The green strength is the initial strength of the composition immediately following application. Green strength can be measured in a number of ways. Accordingly, the green strength is typically expressed in relation to a specific test method. In the present invention, green strength is represented by the storage modulus () as measured by DMA after about 100 seconds as well as by the peel resistance. The hot melt moisture cure composition of the invention exhibits a relatively high green, strength as a function of stiffness, meaning that the Gxe2x80x2 of the composition is at least 25% higher than a hot melt moisture cure composition having the same G*. In preferred embodiments, the  ranges from being about 50%-100% higher than a hot melt moisture cure composition having the same G*. Surprisingly, these properties can be obtained without a corresponding increase in molten viscosity. The storage modulus is typically at least about 1xc3x97104 dynes/cm2 after about 100 seconds of cooling. Preferably, the complex modulus is greater than about 5xc3x97104 dynes/cm2, more preferably greater than about 1xc3x97105 dynes/cm2 and most preferably greater than about 2xc3x97105 dynes/cm2 after about 100 seconds of cooling.
The peel resistance is the displacement of a bond as a function of time. For a given weight, the lower the displacement or the longer the time, the higher the green strength. The hot melt moisture cure adhesive composition typically exhibits a peel resistance of at least about 1 minute, meaning that a 1xe2x80x3 wide bond displaces 20 mm with a 100 g weight. Preferably, the peel resistance is at least about 2 minutes, more preferably at least about 5 minutes, and most preferably about 10 minutes or greater. In the case of fabric bonding the peel resistance at lower weights 10-20 grams tends to more important than the peel resistance at higher weights, 100 grams.
Preferably, the adhesive composition exhibits high heat resistance, as measured by the peel adhesion failure temperature (PAFT). Accordingly, the PAFT is typically greater than about 250xc2x0 F., more preferably greater than about 270xc2x0 F., and more preferably greater than about 290xc2x0 F. xe2x80x9cHot melt moisture cure adhesivexe2x80x9d is an adhesive composition substantially free of solvents and having urethane groups. The composition is substantially solid at room temperature, having a viscosity at 25xc2x0 C. of at least 1xc3x97106 cps. The composition is heated to a temperature ranging from about 100xc2x0 C. to about 200xc2x0 C. and applied in a molten state. The composition physically bonds initially by cooling and also by a moisture induced chemical reaction of isocyanate groups present within urethane prepolymer. It is only subsequent to such chemical curing that the adhesive attains its final characteristics.
A xe2x80x9cpolyurethane prepolymerxe2x80x9d or xe2x80x9cprepolymerxe2x80x9d is an oligourethane having isocyanate groups which is to be regarded as an intermediate on the way to the crosslinked polyurethanes.
The hot melt moisture cure adhesive of the invention comprises at least one polyurethane prepolymer prepared from at least one polyol, at least one polyisocyanate, and at least one high molecular weight thermoplastic polyurethane (TPU) that is substantially free of hard segments. The TPU may be unreactive. However, preferably the TPU has NCO or OH functionality such that it is reacted into the prepolymer of the hot melt moisture cure composition. Thus, in some embodiments, the polyurethane prepolymer and hot melt moisture cure adhesive composition as one in the same. In other embodiments, wherein the TPU is unreactive or wherein other nonreactive ingredients such as tackifiers and thermoplastic polymers are added, the prepolymer is an ingredient of the hot melt moisture cure adhesive composition.
The preparation of isocyanate-terminated polyurethane prepolymers is well known in the art. Prepolymers with residual isocyanate groups are formed by reaction of (1) a mixture of at least one hydroxy functional polymer wherein the total mixture generally has a combined number average molecular weight of about 1,000 to 10,000, preferably of about 1,000 to 6,000, and more preferably of about 2,000 to 5,000 and (2) at least one polyisocyanate, preferably a diisocyanate. The molecular weight of the polyol is selected based on the intended application equipment and substrates to be bonded. Increasing molecular weight makes extrusion of the hot-melt adhesive and penetration into porous or permeable substrates more difficult, whereas decreasing molecular weight can result in the hot melt adhesive being not sufficiently solid at room temperature.
The polyol may be a linear or slightly branched polyester, a polyether or other OH-terminated polymer such as polybutadiene diol. Particular polyesters such as polycaprolactones or polycarbonates may also be used. Preferred prepolymers are based on linear or slightly branched aliphatic or aromatic polyesters containing primary hydroxyl end groups. Other useful polyesters contain secondary hydroxyl or carboxyl end groups. Further, a blend of non-linear and linear polyesters may also be employed.
In general, the selection of polyol(s) is based on the intended end use. For textile bonding and in particular textile bonding with the Nordson Porous Coat(trademark) System, it is preferred that the polyols are substantially amorphous. For the purpose of the present invention xe2x80x9camorphousxe2x80x9d is defined as a component or composition having a heat of fusion of less than about 15 joules/gram. Preferably, the heat of fusion is less than about 10 joules/gram, more preferably less than about 5 joules/gram and most preferably less than about 1 joule/gram. The hot melt moisture cure adhesive of the present invention advantageously attains its high initial strength from the TPU and not from crystallinity. Surprisingly, this approach does not result in a correspondingly high stiffness. However, to prevent blocking at increased line speeds, semi-crystallinine polyols may be advantageous. Further, crystalline polyols are surmised to be suitable for other applications that employ hot melt moisture cure adhesive compositions for bonding plastic, wood, metals, and glass, for example.
The amount of polyol used in the preparation of the hot melt moisture cure adhesive composition typically ranges from about 20 wt-% to about 85 wt-%. Preferably, the polyol component(s) range from about 15 wt-% to about 70 wt-% and most preferably from about 15 wt-% to about 60 wt-%.
Suitable polyether polyols for use in the preparation of the prepolymer include polyalkylene glycol, a linear polyether having 2 OH groups and the general formula HO (xe2x80x94Rxe2x80x94O)m xe2x80x94H, wherein R is a hydrocarbon residue having from 2 to 4 C atoms. Similarily, random and block copolymers are also possible. Specific polyalkylene gylcols for use in the invention include polyethylene glycol, polytetramethylene glycol and polypropylene glycol.
Other suitable polyether polyols for use in the preparation of the prepolymer include compounds that are homopolymers or copolymers formed from one or more alkylene oxide such as ethylene oxide, propylene oxide, 1,2-butylene oxide, 1,4-butylene oxide and mixtures thereof. As in the case of polyalkylene glycol, these polyether polyols may also have a random or block configuration. The higher the oxygen content of the polyol and/or the polyurethane composition, the higher the moisture vapor transmission rate. Accordingly, for adhesive applications wherein breathability is important, polyether polyols are preferred. Employing high molecular weight ethylene oxide as the sole polyol, typically results in a composition that exhibits poor flexibility. However, small concentration of polypropylene oxide or butylene oxide can be added to overcome this deficiency.
Various polyester polyols having two, preferably terminal, OH groups are preferred for use in the formation of the prepolymer. The preparation of polyester polyols from various aromatic acids (such as isophthalic or terephthalic acid) and/or aliphatic acids (such as adipic, acelaic or sebacic acid) and low molecular diols (such as ethylene glycol, butane diol, hexane diol etc) is known. Specific preferred starting materials are typically 1,4-butanediol, 1,6-hexanediol, neopentyl glycol, ethylene glycol, diethylene glycol as well as phthalic anhydride, cyclohexanedicarboxylic, terephthalic, isophthalic, adipic, azelaic, dodecanedioic, dimer, and sebacic acids. Other derivatives that may be employed include e.g., lactones, methyl esters and anhydrides.
The polyesters may be produced in a known manner by melt condensation of the dicarboxylic acids with an excess of the diol component at temperatures in the range from 180xc2x0 C. to 280xc2x0 C. Catalysts or entraining agents, such as toluene for example, may also be used. The polyesters may also be produced by transesterification of dicarboxylic acid esters with diols, such as for example terephthalic acid dimethyl ester with hexane 1,6-diol. The carbonic acid esters are preferably obtained by transesterification of carbonic acid esters, such as for example diphenyl carbonate or carbonic acid diphenyl ester, and an excess of diols.
Whereas the polyols employed for the preparation of the thermoplastic polyurethane are chosen such that the polyurethane is substantially free of hard segments, short chained diols having a molecular weight of less than 600 and preferably less than 400 may optionally be used in the preparation of the hot melt moisture cure adhesive composition. These relatively low molecular weight diols are preferably used in low quantities, for examples up to about 25 wt-%, preferably less than 10 wt-% and most preferably less than 5 wt-% of the total active hydrogen content. Suitable low molecular weight diols optionally used include, di-, tri- and/or tetraethylene glycol, 1,4-dimethylol cyclohexane or reaction products of 4,4xe2x80x2-hydroxyphenyl propane with ethylene and/or propylene oxide. For special effects, it is possible to use diols containing ions and/or structural elements containing ionic groups, such as for example dimethylol propionic acid, N-methyl diethanolamine and/or reaction products of sodium bisuifite and propoxylated butene- 1,4-diol.
Solid or high viscosity liquid polyester glycols are preferred to prevent cold flow of the solid hot melt moisture cure adhesive as well as to improve the initial strength obtained from solidification. As previously discussed, in the case of textile bonding, the polyester polyols are preferably amorphous since crystalline character tends to increase the stiffness of the adhesive coated fabric. However, for applications wherein increased stiffness is not a critical property, crystalline polyols may be employed. Crystalline polyester polyols typically have a melt point from about 40xc2x0 C. to about 120xc2x0 C. and glass transition temperatures, (Tg), of less than about 0xc2x0 C. Preferably, the crystalline polyester polyols are the reaction product of either hexane diol or butane diol, and an acid which may include adipic acid, dodecanedioic acid, sebacic acid, terephthalic acid and mixtures thereof Further, copolymers with other acids and glycols can also be used. Examples of useful polyester polyols include Dynacoll.RTM. 7380 and Dynacoll.RTM. 7381, commercially available crystalline polyester polyols available from CreaNova Inc. in Piscataway, N.J. Polyols having a melt point of less than about 40xc2x0 C. tend to result in compositions that set slowly while the polyols having a melt point of greater than about 120xc2x0 C., on the other hand, require high application temperatures applications. For finished hot melt moisture polyurethane cure compositions having a set time of less than about 20 seconds, crystalline polyester polyols are useful from an amount ranging from about 20% to about 50% by weight of the composition, preferably from about 20% to about 40% by weight and most preferably from about 20% to about 30% by weight of the composition. In general, the higher the percentage of crystalline polyester polyol, the faster the finished composition will set.
The hot melt moisture cure adhesive comprises at least one polyisocyanate present in the prepolymer composition in an equivalent amount greater than that of the hydroxy containing component. The equivalent ratio of isocyanate to hydroxyl is preferably from about 1.2 to about 10 to 1.0 and especially preferably from about 1.6 to 3.0 to 1.0. A xe2x80x9cpolyisocyanatexe2x80x9d is a low molecular weight compound having from 2 to 3 isocyanate groups. Diisocyanates are preferred. Trifunctional isocyanate may be employed at concentrations up to about 10 wt-% provided that inclusion of such does not result in undesirable crosslinking during production and use of the hot melt adhesive:
Suitable polyisocyanates include aliphatic and cycloaliphatic polyisocyanates, aromatic polyisocyanates and-mixtures thereof. Specific examples include toluene diisocyanate (TDI) , diphenylmethane diisocyanate (MDI) and mixtures thereof. Diphenylmethane diisocyanate is understood to be both 4,4xe2x80x2- and 2,4xe2x80x2-diphenylmethane diusocyanate. Preferably, the 2,4xe2x80x2-isomer should not exceed 50% by weight. Other polyisocyanates include naphthylene-1,5-diisocyanate; triphenylmethane-4,4xe2x80x24xe2x80x3-triisocyanate; phenylene-1,3-diisocyanate and -1,4-diisocyanate; dimethyl-3,3xe2x80x2biphenylene-4,4xe2x80x2-diisocyanate; diphenylisopropylidine-4,4xe2x80x2-diisocyanate; biphenylene diisocyanate; xylene -1,3-diisocyanate and xylene -1,4-dilsocyanate. Further, aliphatic diisocyanates are preferred for polyurethane prepolymers with high ultraviolet stability.
Isocyanate-functional derivative(s) of MDI and TDI may be used, such as liquid mixtures of triisocyanate functional derivative with melting point modifiers (e.g., mixtures of MDI with polycarbodiimide adducts such as xe2x80x9cIsonate 143Lxe2x80x9d, commercially available from the Dow Chemical Co., and xe2x80x9cMondur CDxe2x80x9d, commercially available from Mobay Chemical Corp.; small amounts of polymeric diphenylmethane diisocyanates, preferably 10% or less by weight of the total isocyanate component, (e.g., xe2x80x9cPAPIxe2x80x9d, and the series xe2x80x9cPAPI 20xe2x80x9d through xe2x80x9cPAPI 901xe2x80x9d, commercially available from the Dow Chemical Co., xe2x80x9cMondur MRxe2x80x9d, xe2x80x9cMondur MRSxe2x80x9d, and xe2x80x9cMondur MRS-10xe2x80x9d, commercially available from Mobay Chemical Corp., and xe2x80x9cRubinate Mxe2x80x9d, commercially available from ICI Chemicals, Inc.); and blocked isocyanate compounds formed by reacting aromatic isocyanates or the above-described isocyanate-functional derivatives with blocking agents such as ketoximes and the like.
The adhesive composition of the present invention comprises at least one thermoplastic polyurethane (TPU) that is substantially free of hard segments. Typical TPU""s are prepared with low molecular weight diol or polyol chain extenders. The presence of such increases the softening point and melt viscosity of the TPU that in turn increases the strength and toughness. In contrast, the TPU""s for use in the reactive hot melts of the present invention are prepared without low molecular weight chain extenders and thus do not contain urethane hard segments. Although not preferred, incremental amounts of low molecular weight diol/polyol may be employed, provided the resulting adhesive composition exhibits the desired properties of high green strength, reduced stiffness, and preferably low viscosity. In addition to being substantially free of hard segments, the TPU""s for use in the adhesive of the invention are relatively high in molecular weight, having a number average molecular weight (Mn) of at least about 10,000 g/mole and preferably greater than 20,000 g/mole. The number average molecular weight, Mn, is an estimate from the theoretical OH# of the TPU. For example, when made from diols, the Mn≈2xc3x97(56100/TPU OH#).
The TPU is preferably synthesized in accordance with same techniques as used in the preparation of hot melt moisture cure adhesive compositions, with the exception that the TPU in itself is not necessarily NCO terminated and thus, not moisture curing. Rather, the TPU can be prepared such that it is either unreactive or preferably hydroxyl functional such that it becomes reacted into the hot melt moisture cure prepolymer. In preferred embodiments, the polyol for use in the preparation of the TPU is the same as the polyol component added to the TPU in the preparation of the hot melt moisture cure adhesive composition.
The amount of TPU employed in the adhesive composition ranges from about 5wt-% to about 50 wt-% and typically ranges from about 10 wt-% to about 40 wt-% in the reactive hot melt composition. For low viscosity compositions, the amount of TPU preferably ranges from about 10 wt-% to about 30 wt-%. However, for other applications in which higher viscosity compositions are suitable for use, higher amounts and even higher molecular may be employed.
The hot melt moisture cure adhesive composition of the invention can contain other ingredients or adjuvants if desired. For example, fillers (e.g. carbon black, metal oxides such as zinc oxide, and minerals such as talc, clays, silica, silicates, and the like), thermoplastic resins, plasticizers, extending oils antioxidants, pigments, U.V. absorbers, adhesion promoters such as silanes, and the like may be included to impart particular characteristics to the composition. These adjuvants generally can comprise up to 50 weight percent of the composition either individually or in combination.
In addition, the compositions can contain an effective amount of catalyst or reaction accelerator such as tertiary amines, metal-organic compounds, co-curatives such as oxazolidine, and the like. The amount ranges from 0 to 1.5, in particular, from 0.01 to 0.5% by weight based on the weight of prepolymer.
While the compositions of the invention that are best suited by the Nordson Porous Coat(trademark) System are preferably essentially non-phasing, some separation of the polyester components is acceptable for other bonding applications. Moreover, the degree of phasing can be adjusted by varying any or several of certain factors. For example, the degree of chain extension of the polyol, the molecular weight of the polyol and the choice of isocyanate all influence phase separation.
The adhesive compositions of the invention may be prepared by mixing the components at elevated temperature, using conventional mixing techniques. It is preferred to mix the components under anhydrous conditions. Generally, preparation of the adhesive is done without the use of solvents.
The hot melt compositions of the invention achieve their initial, or green, strength through cooling from a liquid state to a solid/semi-solid state and then continue to cure by exposure to water, e.g., water vapor or moisture. High humidity and heat will provide an accelerated rate of cure while low humidity (e.g., 15% R.H. or less) will provide a slower rate of cure.
The polyurethane systems containing isocyanate groups may be produced, for example, by mixing the thermoplastic polyurethane and lower molecular weight polyol, mixing the resulting mixture with an excess of the polyisocyanate and packaging the homogeneous mixture or stirring it until a constant NCO value is obtained, followed by packaging. The reactive polyurethane systems may also be produced continuously in a cascade of stirred tanks or in suitable mixing units, for example in high-speed mixers on the rotor-stator principle. However, the thermoplastic polyurethane and lower molecular weight polyol may also be separately reacted with the polyisocyanate and the prepolymers containing isocyanate groups subsequently mixed in the ratio according to the invention and packaged. In a preferred method of preparation, the thermoplastic polyurethane is made in situ in the reactor, followed by the addition of low molecular weight polyol and additional isocyanate.
Although the low viscosity embodiments are particularly well-suited for porous coating and the bonding of fabrics, the adhesive composition is suitable for a variety""of other hot melt moisture cure adhesive applications. In particular, the hot melt moisture cure composition can be used to bond various plastic and polymerics such as ABS, PVC, plasticized PVC, polycarbonate, polystyrene, EPDM rubber, butyl rubber, polycyclopentadiene, polychloroprene, various block copolymer rubbers such as SIS and SBS, polyamide, nylon, polyolefins, polyester, polyurethanes rubber, polyurethane foam, silicone, polytetrafluoroethylene, ethylene vinyl acetate, foamed EVA, etc. Further, the adhesive composition is suitable for bonding painted surfaces (e.g. epoxy polyester, TGIC, polyurethane, nitocellulose laquere), metal bonding (e.g. cold rolled steel, galvanized steel, phosphatized steel, aluminium, anodized aluminum, stainless steel, copper, magnesium, brass) glass, cellulose and cellulose composites (e.g. plywood, particle board, high pressure laminant, medium density fiberboard, orinetated strandboard, sponge, cellulose sponge) as well as other composites such as fiberglass, graphite and kevlar.
The invention is further illustrated by the following non-limiting examples.
Test Methods
1. DMA Testingxe2x80x94The adhesive sample is heated for about 30 minutes at 250xc2x0 F. and applied to bottom plate of parallel plate rheometer equilibrated at 25xc2x0 C. The top plate is pressed onto the sample forming about a 1 mm gap and the sample is tested at 1 radians/second as the sample cools and sets at 25xc2x0 C. Various measurements such as the storage modulus, Gxe2x80x2; the loss modulus, Gxe2x80x3; the complex modulus, G*, [the square root of the sum of (Gxe2x80x2)2 +(Gxe2x80x3)2]; and the tan xcex4, (ratio of the loss modulus divided by the storage modulus) is recorded as function of time as the material sets. A temperature sweep at 10 radians/seconds is employed for the purpose of measuring the flow properties of the material in its molten state.
2. Brookfield Viscosityxe2x80x94The molten viscosity is determined with Brookfield models DVH, DV-II or DV-III. An appropriate spindle size and hot melt adhesive sample size was selected in accordance with the viscometer manufacturers instructions. The adhesive sample was melted in the thermocel at the temperature at which the viscosity was to be measured. The spindle was lowered into the melted adhesive sample. The motor was turned on at the lowest speed and the corresponding torque reading displayed. The speed was increased until the torque reading stabilized and the viscosity measurement recorded after 30 minutes.
3. Peel Resistancexe2x80x94A adhesive sample is heated and coated onto a 1xe2x80x3 wide aluminum strip using a heated draw down bar with a 0.005xe2x80x3 gap. The aluminum strip is placed on a 120xc2x0 C. hot plate or in a 120xc2x0 C. oven for one minute. The strip is removed from the hot plate or oven and a 1xe2x80x3 wide piece of canvas is applied to the strip and then immediately hand rolled to form a bond. The aluminum strip is attached vertically to a holder and after a given period of time, a weight is attached to one end of the fabric forming a 180xc2x0 peel angle. The cooling time before applying the weight and the displacement verses time is recorded for the material.
4. Peel Adhesive Failure Temperature (PAFT)xe2x80x94The adhesive was heated to 250xc2x0 F. and applied to #10 canvas with a 10 mil heated drawdown bar. Two 1xe2x80x3 wide pieces of canvas were mated and rolled having a total adhesive thickness of 20 mils. The samples were cured for 2-3 weeks in a 75xc2x0 F./50% relative humidity chamber. After curing, the samples were placed in a programmed oven with 1 kg weights, ramping the temperature up from 25xc2x0 C. to 175xc2x0 C. at a rate of 25xc2x0 C. per hour. The oven automatically records the temperature at which the samples fail. The reported result is the average of 5-7 samples.
Examples 1, 4, 9, 11, 13 and 20 describe the preparation of high molecular weight thermoplastic polyurethanes that are free of hard segments. The thermoplastic polyurethanes are then employed as reactants in the preparation of the hot melt moisture cure adhesive compositions of Examples 2-3, 5-6, 8, 10, 12, 14 and 21. Examples 7, 15-19 and 22 illustrate the preparation of the high molecular weight thermoplastic polyurethanes that are free of hard segments in situ during the synthesis of the reactive adhesive composition. A variety of crystalline and amorphous polyols are employed. Examples 2-3, 5-7, 15 and 17 are prepared solely with amorphous components, whereas the remaining examples contain at least one crystalline component in the preparation of the TPU, the preparation of the hot melt moisture cure adhesive composition, or both.